Specific Absorption Rate Patterns Research Articles

Simultaneous estimation of SAR, thermal diffusivity, and damping using periodic power modulation for MRgFUS quality assurance

Purpose: A crucial aspect of quality assurance in thermal therapy is periodic demonstration of the heating performance of the device. Existing methods estimate the specific absorption rate (SAR) from the temperature rise after a short power pulse, which yields a biased estimate as thermal diffusion broadens the apparent SAR pattern. To obtain an unbiased estimate, we propose a robust frequency-domain method that simultaneously identifies the SAR as well as the thermal dynamics. Methods: We propose a method consisting of periodic modulation of the FUS power while recording the response with MR thermometry (MRT). This approach enables unbiased measurements of spatial Fourier coefficients that encode the thermal response. These coefficients are substituted in a generic thermal model to simultaneously estimate the SAR, diffusivity, and damping. The method was tested using a cylindrical phantom and a 3 T clinical MR-HIFU system. Three scenarios with varying modulation strategies are chosen to challenge the method. The results are compared to the well-known power pulse technique. Results: The thermal diffusivity is estimated at 0.151 mm2s–1 with a standard deviation of 0.01 mm2s–1 between six experiments. The SAR estimates are consistent between all experiments and show an excellent signal-to-noise ratio (SNR) compared to the well established power pulse method. The frequency-domain method proved to be insensitive to B 0-drift and non steady-state initial temperature distributions. Conclusion: The proposed frequency-domain estimation method shows a high SNR and provided reproducible estimates of the SAR and the corresponding thermal diffusivity. The findings suggest that frequency-domain tools can be highly effective at estimating the SAR from (biased) MRT data acquired during periodic power modulation.

ODP638 Towards the development of treatment planning for microwave thermal ablation of adrenal adenomas

Abstract Primary aldosteronism (PA) is the principal cause of secondary hypertension and accounts for 5-12% of all hypertension. PA is characterized by unregulated aldosterone secretion from unilateral or bilateral nodules on the adrenal glands. High levels of aldosterone are associated with an increased risk of cardiovascular and cerebrovascular complications. Conventional therapy includes adrenalectomy and mineralocorticoid receptor antagonists. These carry the challenges of surgical complication and poor tolerability to medication side effects. Moreover, no definitive therapy exists for bilateral PA driven by nodular disease. Microwave thermal ablation (MTA) offers a percutaneous alternative to adrenalectomy for PA and with precision engineering of microwave antennae may also offer a definitive option for bilateral PA. MTA can be applied to select and localize therapy to aldosterone producing adenomas while preserving the surrounding functional adrenocortical tissue. Advance treatment planning is essential to optimize the following parameters (i) microwave power delivery, (ii) probe placement, (iii) heat-sink minimisation. We present a novel and state-of-the-art simulated treatment planning approach to adrenal MTA through 3D anatomic reconstruction and dielectric electromagnetic modeling to predict therapeutic response. MTA consists of a microwave applicator antenna to transfer the electromagnetic (EM) energy and induce cytotoxic hyperthermia. The EM power distribution across tissue is calculated by solving the Maxwell's equations. The Finite-Difference Time-Domain (FDTD) method computes the SAR (Specific Absorption Rate) patterns and the Pennes’ bio heat equation models the bioheat transfer within the adrenal gland. Treatment planning combines CT abdomen and 11C-metomidate PET/CT images to generate a 3D patient specific model using a segmentation routine in iSeg (version 3.1, Speag, Zurich, Switzerland)) which is combined with the applicator model in Sim4Life (version 6.2, Speag, Zurich, Switzerland). Electrical and thermal properties of the organ are assigned and the microwave power is modelled to optimally and selectively heat the target tissue while minimizing damage to adjacent cortical tissue. We present results from 3D computational modelling derived from patient imaging-scans employed to predict the thermal ablation zone in patients affected by PA. Using this system, anatomically, dielectrically and thermally accurate models of adrenal glands, nodules and adjacent sensitive structures were reconstructed and energy delivery strategies were evaluated. This method was used to assess the ability to inform appropriate power settings and probe placement for ablating adrenal gland nodules. Treatment planning not only represents an important tool to precisely deliver controlled doses of microwave thermal therapy in the context of adrenal ablation but also represents an important tool to inform future applicator probe design and appropriate imaging. Presentation: Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.

Transient changes during microwave ablation simulation : a comparative shape analysis

Microwave ablation therapy is a hyperthermic treatment for killing cancerous tumours whereby microwave energy is dispersed into a target tissue region. Modelling can provide a prediction for the outcome of ablation, this paper explores changes in size and shape of temperature and Specific absorption rate fields throughout the course of simulated treatment with different probe concepts. Here, an axisymmetric geometry of a probe embedded within a tissue material is created, solving coupled electromagnetic and bioheat equations using the finite element method, utilizing hp discretisation with the NGSolve library. Results show dynamic changes across all metrics, with different responses from different probe concepts. The sleeve probe yielded the most circular specific absorption rate pattern with circularity of 0.81 initially but suffered the largest reduction throughout ablation. Similarly, reflection coefficients differ drastically from their initial values, with the sleeve probe again experiencing the largest change, suggesting that it is the most sensitive the changes in the tissue dielectric properties in these select probe designs. These collective characteristic observations highlight the need to consider dielectric property changes and probe specific responses during the design cycle.

Direct SAR mapping by thermoacoustic imaging: A feasibility study.

To develop a new method capable of directly measuring specific absorption rate (SAR) deposited in tissue using the thermoacoustic signal induced by short radiofrequency (RF) pulse excitation. A detailed model based on the thermoacoustic wave generation and propagation is presented. We propose a new concept for direct measurement of SAR, to be used as a safety assessment/monitoring tool for MRI. The concept involves the use of short bursts of RF energy and the measurement of the resulting thermoacoustic excitation pattern by an array of ultrasound transducers, followed by image reconstruction to yield the 3D SAR distribution. We developed a simulation framework to model this thermoacoustic SAR mapping concept and verified the concept in vitro. Simulations show good agreement between reconstructed and original SAR distributions with an error of 4.2, 7.2, and 8.4% of the mean SAR values in axial, sagittal, and coronal planes and support the feasibility of direct experimental mapping of SAR distributions in vivo. The in vitro experiments show good agreement with theory (r2 = 0.52). A novel thermoacoustic method for in vivo mapping of local SAR patterns in MRI has been proposed and verified in simulation and in a phantom experiment. Magn Reson Med 78:1599-1606, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Development and validation of a MRgHIFU non-invasive tissue acoustic property estimation technique

MR-guided high-intensity focussed ultrasound (MRgHIFU) non-invasive ablative surgeries have advanced into clinical trials for treating many pathologies and cancers. A remaining challenge of these surgeries is accurately planning and monitoring tissue heating in the face of patient-specific and dynamic acoustic properties of tissues. Currently, non-invasive measurements of acoustic properties have not been implemented in MRgHIFU treatment planning and monitoring procedures. This methods-driven study presents a technique using MR temperature imaging (MRTI) during low-temperature HIFU sonications to non-invasively estimate sample-specific acoustic absorption and speed of sound values in tissue-mimicking phantoms. Using measured thermal properties, specific absorption rate (SAR) patterns are calculated from the MRTI data and compared to simulated SAR patterns iteratively generated via the Hybrid Angular Spectrum (HAS) method. Once the error between the simulated and measured patterns is minimised, the estimated acoustic property values are compared to the true phantom values obtained via an independent technique. The estimated values are then used to simulate temperature profiles in the phantoms, and compared to experimental temperature profiles. This study demonstrates that trends in acoustic absorption and speed of sound can be non-invasively estimated with average errors of 21% and 1%, respectively. Additionally, temperature predictions using the estimated properties on average match within 1.2 °C of the experimental peak temperature rises in the phantoms. The positive results achieved in tissue-mimicking phantoms presented in this study indicate that this technique may be extended to in vivo applications, improving HIFU sonication temperature rise predictions and treatment assessment.

Optimal design of focused arrays for microwave‐induced hyperthermia

This study studies optimal design of focused antenna arrays for microwave-induced hyperthermia. The phases and amplitudes of the driving signals of array elements are determined by optimising the power transmission efficiency of the system consisting of the transmitting antenna array, a receiving antenna located in the target area, and an equivalent phantom representative of human tissues. On the basis of the proposed technique, a 4 × 2 focused patch array is designed to operate on a fat-mimicking phantom with working frequency at 433 MHz. The measured patterns for specific absorption rate indicate that the energy is focused in the desired spot and are in close agreement with simulations, which validate the optimisation method in a medium with low permittivity and low loss. A 4 × 3 focused patch array operating on a human body phantom with same working frequency is then designed, in which an air space and two additional dielectric layers have been introduced between the planar array and the body-mimicking phantom for impedance matching. A good focal performance at desired spot is also achieved, and the simulated and measured specific absorption rate patterns agree very well. This demonstrates the validity of the proposed design method in a medium with high permittivity and high loss.

Development of a novel method to enhance the therapeutic effect on tumours by simultaneous action of radiation and heating

Purpose: This paper describes the development of a new type of electromagnetic hyperthermia applicator delivering dose control within large application fields and increased effectiveness by providing simultaneous action of radiation and heating (SRH) in malignant tumours, and development of a dosimetric feedback method to support SRH. Materials and methods: Single and phased arrays of flexible applicators have been developed to allow simultaneous hyperthermia and external beam therapy. A frequency of 434 MHz is used to heat near-surface and moderately deep-seated tumours and 70 MHz for deep-seated tumours. Phase and amplitude control allows focusing of electromagnetic energy (EM) to deep-seated tumours. The specific absorption rate (SAR) dose distribution can be modified to achieve uniform heating of tumours with complex shapes and heterogeneous tissue properties. A lithium fluoride thermoluminescent dosimeter (TLD) in a flexible film cassette has been developed for real-time dose measurement. Results: Four types of 434 MHz applicators were manufactured with 3, 4, 9 or 12 independent applicators. Two types of 70 MHz applicators were made with 4 or 6 independent applicators. Phantom tests demonstrated the ability to control the SAR pattern by phase and amplitude control. Placement of the dosimeter between bolus and phantom increased the phantom surface temperature up to 3 °C and showed that the ratio of absorbed energy in TLD to dose in water approaches (0.83 ± 3%) for photon energies >60 keV. Conclusions: Simultaneous and controlled radiation and local hyperthermia is technically feasible in a preclinical setting, a clinical feasibility test is the next step.

A Balun-Free Helical Antenna for Minimally Invasive Microwave Ablation

We present a balun-free coax-fed helical antenna for microwave ablation. The proposed antenna produces a localized specific absorption rate pattern at the desired frequency of operation without using a coaxial balun. This reduces the outer diameter of the antenna and, thus, its invasiveness. Balun-free operation of the proposed antenna is achieved by operating the helical antenna at the second resonant frequency of the helix where a current minimum occurs at the feed point, resulting in an intrinsically high feed-point impedance. This efficiently chokes the currents excited on the outer surface of the feeding cable. Using a compact quarter-wavelength impedance transformer or a coaxially implemented pi network of reactive elements at the antenna feed point, we achieve an excellent impedance match between the antenna and the main feeding line. We fabricated a prototype of the proposed helical antenna with an outer diameter of 2.2 mm using the pi matching network. We used this prototype to conduct ablation experiments in ex vivo bovine liver. The dimensions of the resulting ablation zones are similar to those produced by coaxially fed antennas that use coaxial baluns. Our balun-free antenna design offers a promising solution for reducing the invasiveness of interstitial antennas used in microwave ablation.

SAR reduction in 7T C-spine imaging using a "dark modes" transmit array strategy.

Local specific absorption rate (SAR) limits many applications of parallel transmit (pTx) in ultra high-field imaging. In this Note, we introduce the use of an array element, which is intentionally inefficient at generating spin excitation (a "dark mode") to attempt a partial cancellation of the electric field from those elements that do generate excitation. We show that adding dipole elements oriented orthogonal to their conventional orientation to a linear array of conventional loop elements can lower the local SAR hotspot in a C-spine array at 7 T. We model electromagnetic fields in a head/torso model to calculate SAR and excitation B1 (+) patterns generated by conventional loop arrays and loop arrays with added electric dipole elements. We utilize the dark modes that are generated by the intentional and inefficient orientation of dipole elements in order to reduce peak 10g local SAR while maintaining excitation fidelity. For B1 (+) shimming in the spine, the addition of dipole elements did not significantly alter the B1 (+) spatial pattern but reduced local SAR by 36%. The dipole elements provide a sufficiently complimentary B1 (+) and electric field pattern to the loop array that can be exploited by the radiofrequency shimming algorithm to reduce local SAR.

Clinical integration of software tool VEDO for adaptive and quantitative application of phased array hyperthermia in the head and neck

Background and purpose: In Rotterdam, patient-specific hyperthermia (HT) treatment planning (HTP) is applied for all deep head and neck (H&N) HT treatments. In this paper we introduce VEDO (the Visualisation Tool for Electromagnetic Dosimetry and Optimisation), the software tool required, and demonstrate its value for HTP-guided online complaint-adaptive (CA) steering based on specific absorption rate (SAR) optimisation during a H&N HT treatment.Materials and methods: VEDO integrates CA steering, visualisation of the SAR patterns and mean tumour SAR (SARtarget) optimisation in a single screen. The pre-calculated electromagnetic fields are loaded into VEDO. During treatment, VEDO shows the SAR pattern, overlaid on the patients’ CT-scan, corresponding to the actually applied power settings and it can (re-)optimise the SAR pattern to minimise SAR at regions where the patient senses discomfort while maintaining a high SARtarget.Results: The potential of the quantitative SAR steering approach using VEDO is demonstrated by analysis of the first treatment in which VEDO was used for two patients using the HYPERcollar.These cases show that VEDO allows response to power-related complaints of the patient and to quantify the change in absolute SAR: increasing either SARtarget from 96 to 178 W/kg (case 1); or show that the first SAR distribution was already optimum (case 2).Conclusion: This analysis shows that VEDO facilitates a quantitative treatment strategy allowing standardised application of HT by technicians of different HT centres, which will potentially lead to improved treatment quality and the possibility of tracking the effectiveness of different treatment strategies.

Thermal characteristics of thermobrachytherapy surface applicators for treating chest wall recurrence

The aim of this study was to investigate temperature and thermal dose distributions of thermobrachytherapy surface applicators (TBSAs) developed for concurrent or sequential high dose rate (HDR) brachytherapy and microwave hyperthermia treatment of chest wall recurrence and other superficial diseases. A steady-state thermodynamics model coupled with the fluid dynamics of a water bolus and electromagnetic radiation of the hyperthermia applicator is used to characterize the temperature distributions achievable with TBSAs in an elliptical phantom model of the human torso. Power deposited by 915 MHz conformal microwave array (CMA) applicators is used to assess the specific absorption rate (SAR) distributions of rectangular (500 cm2) and L-shaped (875 cm2) TBSAs. The SAR distribution in tissue and fluid flow distribution inside the dual-input dual-output (DIDO) water bolus are coupled to solve the steady-state temperature and thermal dose distributions of the rectangular TBSA (R-TBSA) for superficial tumor targets extending 10–15 mm beneath the skin surface. Thermal simulations are carried out for a range of bolus inlet temperature (Tb = 38–43 °C), water flow rate (Qb = 2–4 L min−1) and tumor blood perfusion (ωb = 2–5 kg m−3 s−1) to characterize their influence on thermal dosimetry. Steady-state SAR patterns of the R- and L-TBSA demonstrate the ability to produce conformal and localized power deposition inside the tumor target sparing surrounding normal tissues and nearby critical organs. Acceptably low variation in tissue surface cooling and surface temperature homogeneity was observed for the new DIDO bolus at a 2 L min−1 water flow rate. Temperature depth profiles and thermal dose volume histograms indicate bolus inlet temperature (Tb) to be the most influential factor on thermal dosimetry. A 42 °C water bolus was observed to be the optimal choice for superficial tumors extending 10–15 mm from the surface even under significant blood perfusion. Lower bolus temperature may be chosen to reduce the thermal enhancement ratio (TER) in the most sensitive skin where maximum radiation dose is delivered and to extend the thermal enhancement of radiation dose deeper. This computational study indicates that well-localized elevation of tumor target temperature to 40–44 °C can be accomplished by large surface-conforming TBSAs using appropriate selection of coupling bolus temperature.

Compact Patch Antenna for Electromagnetic Interaction With Human Tissue at 434 MHz

Single element loop, dipole and conventional square patch antennas have been used as hyperthermia applicators in the treatment of cancerous human cells at superficial depths inside the body. A smaller novel patch antenna in very close proximity to a phantom tissue model produces an enhanced specific absorption rate pattern without significant frequency detuning or impedance mismatch. The new patch increases its coupling aperture by supporting a combination of resonances that are also typical for loop, dipole and square patch antennas. For computation efficiency and clarity in the synthesized hyperthermia treatment conditions, simplified planar tri-layered tissue models interfaced with a water-bolus are used to study the permittivity loading on the antennas and the resultant specific absorption rates.

Patient positioning in deep hyperthermia: influences of inaccuracies, signal correction possibilities and optimization potential

In this deep hyperthermia study, the robustness of SAR (specific absorption rate) patterns to patient-position variations is assessed, as well as the possibilities to correct for improper positioning and the benefits of non-standard positions. With a finite element model, the SAR distributions were predicted for ten patients at 33 positions. Position sensitivity is assessed for both SAR-focus steering, i.e. settings based on a calculated focus in a cylindrical patient representation, and HTP (hyperthermia treatment planning)-guided steering, i.e. model-based optimization of the SAR distribution. Position inaccuracies of less than 1 cm do not significantly affect SAR patterns. For SAR-focus steering, the SAR maximum is not always at the desired focus location, especially in the Y (anterior/posterior)- and Z (axial)-directions. For a maximum shift of 5 cm in all directions, both SAR-focus steering and HTP-guided steering are suitable to correct for improper positioning up to the level that none of the investigated positions appears preferable. Current positioning precision is sufficient in the X (right–left)-direction, but precision measurements are needed to reach the desired accuracy in the Y-direction. In the Z-direction, a cranial shift of the applicator is predicted to be beneficial. If the position is known accurately, correction of the treatment setting is possible without loss of heating efficiency. Additionally, no preferable positions exist.

An ultrasound cylindrical phased array for deep heating in the breast: theoretical design using heterogeneous models

The objective of this theoretical study is to design an ultrasound (US) cylindrical phased array that can be used for hyperthermia (40–44 °C) treatment of tumours in the intact breast. Simultaneously, we characterize the influence of acoustic and thermal heterogeneities on the specific absorption rate (SAR) and temperature patterns to determine the necessity of using heterogeneous models for a US applicator design and treatment planning. Cylindrical configurations of monopole transducers are studied on their ability to generate interference patterns that can be steered electronically to the location of the target region. Hereto, design parameters such as frequency, number of transducers per ring, ring distance and number of rings are optimized to obtain a small primary focus, while suppressing secondary foci. The models account for local heterogeneities in both acoustic (wave velocity and absorption) and thermal (blood perfusion rate, heat capacity and conductivity) tissue properties. We used breast models with a central tumour (30 × 20 × 38 mm3) and an artificial thorax tumour (sphere with a radius of 25 mm) to test the design. Simulations predict that a US cylindrical phased array, consisting of six rings with 32 transducers per ring, a radius of 75 mm and 66 mm distance between the first and sixth transducer ring, operating at a frequency of 100 kHz, can be used to obtain 44 °C in the centre of tumours located anywhere in the intact breast. The dimensions of the volumes enclosed by the 41 °C iso-temperature are 19 × 19 × 21 mm3 and 21 × 21 × 32 mm3 for the central and the thorax tumours, respectively. It is demonstrated that acoustic and thermal heterogeneities do not disturb the SAR and temperature patterns.

Electromagnetic Head-And-Neck Hyperthermia Applicator: Experimental Phantom Verification and FDTD Model

To experimentally verify the feasibility of focused heating in the neck region by an array of two rings of six electromagnetic antennas. We also measured the dynamic specific absorption rate (SAR) steering possibilities of this setup and compared these SAR patterns to simulations. Using a specially constructed laboratory prototype head-and-neck applicator, including a neck-mimicking cylindrical muscle phantom, we performed SAR measurements by electric field, Schottky-diode sheet measurements and, using the power-pulse technique, by fiberoptic thermometry and infrared thermography. Using phase steering, we also steered the SAR distribution in radial and axial directions. All measured distributions were compared with the predictions by a finite-difference time-domain-based electromagnetic simulator. A central 50% iso-SAR focus of 35 +/- 3 mm in diameter and about 100 +/- 15 mm in length was obtained for all investigated settings. Furthermore, this SAR focus could be steered toward the desired location in the radial and axial directions with an accuracy of approximately 5 mm. The SAR distributions as measured by all three experimental methods were well predicted by the simulations. The results of our study have shown that focused heating in the neck is feasible and that this focus can be effectively steered in the radial and axial directions. For quality assurance measurements, we believe that the Schottky-diode sheet provides the best compromise among effort, speed, and accuracy, although a more specific and improved design is warranted.

The HYPERcollar: A novel applicator for hyperthermia in the head and neck

The purpose of this work was to define all features, and show the potential, of the novel HYPERcollar applicator system for hyperthermia treatments in the head and neck region. The HYPERcollar applicator consists of (1) an antenna ring, (2) a waterbolus system and (3) a positioning system. The specific absorption rate (SAR) profile of this applicator was investigated by performing infra-red measurements in a cylindrical phantom. Mandatory patient-specific treatment planning was performed as an object lesson to a patient with a laryngeal tumour and an artificial lymph node metastasis. Comfort tests with healthy volunteers have revealed that the applicator provides sufficient comfort to maintain in treatment position for an hour: the standard hyperthermia treatment duration in our centre. By phantom measurements, we established that a central focus in the neck can be obtained, with 50% iso-SAR lengths of 3.5 cm in transversal directions (x/y) and 9–11 cm in the axial direction (z). Using treatment planning by detailed electromagnetic simulations, we showed that the SAR pattern can be optimised to enable simultaneous encompassing of a primary laryngeal tumour and a lymph node metastasis at the 25% iso-SAR level. This study shows that the applicator enables a good control, and sufficient possibilities for optimisation, of the SAR pattern. In an ongoing clinical feasibility study, we will investigate the possibilities of heating various target regions in the neck with this apparatus.

A Floating Sleeve Antenna Yields Localized Hepatic Microwave Ablation

We report a novel coaxial antenna for hepatic microwave ablation. This device uses a floating sleeve, that is, a metal conductor electrically isolated from the outer connector of the antenna coaxial body, to achieve a highly localized specific absorption rate pattern that is independent of insertion depth. This floating sleeve coaxial dipole antenna has low power reflection in the 2.4-GHz IMS band. Ex vivo experiments confirm our numerical simulation results. Index Terms-Ablation, coaxial aperture antennas, finite element methods, floating sleeve, microwave heating.

An alternative method for determination of low-frequency specific absorption rate patterns in homogeneous phantoms

We propose a new application of voltage gradient measurements to determine specific absorption rate (SAR) at low frequencies where quasi-static electromagnetic conditions apply. This method, which we call the voltage gradient method, relies on direct measurement of the voltage field rather than measurement of the electric field or thermal transients. The voltage gradient method is fast and can be implemented with voltmeters of moderate cost. We tested the voltage gradient method using normal saline, in a phantom with simple geometry, and a sine wave voltage source at 5, 10, 20 and 50 kHz. Compared to the SAR measured thermally in the same phantom, the voltage gradient method produced almost identical curves when normalized. When the results of the voltage gradient method were scaled to the same power level used for the thermal SAR, the agreement was compatible with typical thermal SAR accuracy.

Feasibility of salvage interstitial microwave thermal therapy for prostate carcinoma following failed brachytherapy: studies in a tissue equivalent phantom

Thermal therapy is an experimental treatment to destroy solid tumours by heating them to temperatures ranging from 55 °C to 90 °C, inducing thermal coagulation and necrosis of the tumour. We are investigating the feasibility of interstitial microwave thermal therapy as a salvage treatment for prostate cancer patients with local recurrence following failed brachytherapy. Due to the electrical and thermal conductivity of the brachytherapy seeds, we hypothesized that the seeds could scatter the microwave energy and cause unpredictable heating. To investigate this, a 915 MHz helical antenna was inserted into a muscle-equivalent phantom with and without brachytherapy seeds. Following a 10 W, 5 s input to the antenna, the temperature rise was used to calculate absorbed power, also referred to as specific absorption rate (SAR). Plane wave models based on Maxwell's equations were also used to characterize the electromagnetic scattering effect of the seeds. In addition, the phantom was heated with 8 W for 5 min to quantify the effect of the seeds on the temperature distribution during extended heating. SAR measurements indicated that the seeds had no significant effect on the shape and size of the SAR pattern of the antenna. However, the plane wave simulations indicated that the seeds could scatter the microwave energy resulting in hot spots at the seed edges. Lack of experimental evidence of these hot spots was probably due to the complex polarization of the microwaves emitted by the helical antenna. Extended heating experiments also demonstrated that the seeds had no significant effect on the temperature distributions and rates of temperature rise measured in the phantom. The results indicate that brachytherapy seeds are not a technical impediment to interstitial microwave thermal therapy as a salvage treatment following failed brachytherapy.

SAR pattern perturbations from resonance effects in water bolus layers used with superficial microwave hyperthermia applicators

This study examines the effect of various thickness water bolus coupling layers on the SAR (Specific Absorption Rate) patterns from Dual Concentric Conductor (DCC) based Conformal Microwave Array (CMA) superficial hyperthermia applicators. Previous theory has suggested that water bolus coupling layers can be considered as a dielectric resonator; therefore, it is possible for the impinging electric field to stimulate volume oscillations and surface wave oscillations inside the water bolus. These spurious oscillations will destructively or constructively interact with the impinging electric field to cause a perturbation of the applicator SAR pattern. An experiment was designed which consisted of mapping the electric field produced by a four element DCC CMA applicator in liquid muscle phantom at depths of 5 and 10mm in front of four different thickness water boli; 0 (no bolus) 4, 9 and 13mm. Using the Finite Difference Time Domain (FDTD) method, SAR distributions were calculated for similar test cases. It was found that for water bolus thicknesses of 9mm or greater, there is a marked perturbation of both experimental and theoretical SAR distributions. It is believed that this perturbation is experimental confirmation of the volume and surface wave oscillation theory described by previous investigators.